Posted by: bkivey | 23 November 2010

Winter Weather

In this part of the world the seasonal rains usually start around the end of Otober and the first real cold snap comes about a month later. Snow, which usually starts in earnest around the middle of November in the mountains, is more problematic in the valleys. We usually get a couple of light dustings in December or January, and most people are fine with that. If I want to see snow, I know where to find it.

There are exceptions. There was the Storm of 2004 which no one predicted and brought two straight days of heavy snow and sleet to the Willamette Valley and Portland metro area. In December 2008 we had upwards of 16″ of snow in a fifteen day period, and there was a brief, intense snow fall in December of last year which was unpredicted and  brought Portland to a halt.

Meteorologists say that Portland’s proximity to two mountain ranges, an ocean, and a large river in a deep gorge make winter precipitation difficult to predict, so they tend to overpromise and underdeliver on winter storm warnings. This is preferable to the alternative, but it does tend to get people excited and apprehensive. Our first winter storm warning this year was a good example of how this usually works. 

In this and subsequent images the time stamp is in the upper left corner. My location is the circled cross in the center of the image.  The key shows precipitation reflectivity in decibels: the higher the number, the more intense the precipitation. The alphanumeric symbols are storm cells with tracks and speed shown as vectors. Yellow diamonds are mesocyclones.

The prediction at this time was for 1 – 3 inches of snow in the valley with more at higher elevations around the city. Snowfall was predicted to start around 1400 as cold air came south with snow showers throughout the day and a winter storm warning in effect until 2200. From this image captured at 0900 we see lots of rain in Oregon with a ‘wintry mix’ (snow mixed with rain) further north and snow in and around Olympia.

At 1300 it’s still raining pretty good in Oregon but the frozen precipitation has moved further north. What the image doesn’t show is that during the day the wind was coming out of the SW, bringing in warmer (well, not as cold) air from the south and pushing the cold air away from the Portland area.

It’s 1700, and no snow in Portland, but still plenty of rain, and that’s diminishing. The cold air is still well to the north.

2100, and the cold air has arrived. As late as 1900 the temperature in the area was still above freezing. Shortly after this image was captured we did get snow for about fifteen minutes; just enough to bring out every kid in the neighborhood. As brief as it was, I think that this is the earliest I’ve seen snow in this area.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s


%d bloggers like this: